Referring to FIGS. 1 and 2 for the exploded view and the cross-sectional view of a traditional built-in three-speed hub respectively, the traditional hub includes an axle shaft 1, a hollow planetary gear set 2 passing through and connecting the middle of the axle shaft 1, a clutch 3 having a plurality of teeth disposed at an end of the clutch 3 for engaging an interior of a driving member 4, an external ring 5 disposed at the external periphery of the planetary gear set 2 and having two corresponding ratchets 5b, a hub 7 having a plurality of internal teeth 8 disposed on a sidewall of the hub 7, and a driving member 4 having its external periphery engaged with a bicycle sprocket 9, such that if the sprocket 9 drives the driving member 4 to rotate, the clutch 3 situated at the driving member 4 will be driven by the clutch 3, and a ring gear 3a in the clutch 3 will be latched with the internal teeth of the planetary gear set 2 for driving the planetary gear set 2 to rotate, and four planetary gear wheels 2a drive the external ring 5 to rotate, and two protruded teeth 5a of the external ring 5 are engaged with the internal teeth 8 disposed at a side of the hub 7 to rotate the hub 7. If the clutch 3 is withdrawn and separated from the planetary gear set 2, an end of each ratchet 5b of the external ring 5 will be pushed outward, so that another embedding end at the external side is hidden and will not drive the hub 7 to rotate, and a plurality of wheel teeth 4a of the driving member 4 are engaged with the protruded teeth 5a of the external ring 5 for driving the external ring 5 to rotate, and four planetary gear sets 2 in the external ring 5 can be rotated, and the embedding teeth 2b can be engaged with the internal teeth 8 at the other side of the hub 7, so that the hub 7 can be rotated. The movement of the clutch 3 is controlled by the displacement of a gear lever 1a in the axle shaft 1. When the slope of a skewed groove 1b of the axle shaft 1 guides the ring gear 3a at the internal side of the clutch 3 for a connection, the ring gear 3a is guided slantingly into the skewed groove 1b during a rotation, so that the driving member 4 can drive the external ring 5. When the gear lever 1a is pushed inward again, the clutch 3 pushes the planetary gear set 2, and the embedding teeth 2b are engaged with the internal teeth 8 of the hub 7 to drive the hub 7 to rotate.
Since the foregoing internal teeth 8 disposed on both sides of a traditional hub 7 are in the hub 7, the process for manufacturing the teeth is not easy, and the skewed groove 1b of the axle shaft 1 is in a spiral shape which makes the manufacturing process more difficult. Further, the prior art speed-change hub consists many complicated components, and thus causing a heavy total weight and incurring a high manufacturing cost.